1. Field of the Invention
The present invention relates to a direct current (DC) motor control device of a water heater and, more particularly, to a control device, which adopts a DC motor having a large torque and utilizes a modified low-cost phase control method to change the output voltage.
2. Description of the Prior Art
As shown in FIGS. 1 and 2, a conventional motor of a water heater includes a fanner control system 11, a rotation speed detection circuit 12, and a micro-processing unit 13. The fanner control system 11 includes an alternating current (AC) motor 111 and a control unit 112. The control unit 112 is connected to the AC motor 111. The control unit 112 includes a first control circuit 1121 and a second control circuit 1122.
The first control circuit 1121 consists of a resistor R11 and a capacitor C11. The first control circuit 1121 is used to filter out noises from the input AC voltage source Vs1.
The second control circuit 1122 consists of resistors R12˜R15, capacitors C12 and C13, an inductor L1, a transistor Q1, a bi-directional thyristor (semiconductor-controlled rectifier) TR1, and a photo coupler PC1. The photo coupler PC1 is principally used to isolate the AC source from the DC source so as to prevent interferences and short circuits therebetween. The photo coupler PC1 is to conduct the bi-directional thyristor TR1.
The input of the rotation speed detection circuit 12 is connected to the AC motor 111 of the fanner control system 11, and to detect the rotation speed of the AC motor 111.
The input of the micro-processing unit 13 is connected to the rotation speed detection circuit 12, and the output thereof is connected to the second control circuit 1122 of the control unit 112. The micro-processing unit 13 is used to process outputs of the rotation speed detection circuit 12 and then output a control signal to the second control circuit 1122.
The way of operation for the above circuit is described below. The rotation speed of the AC motor 111 of the fanner control system 11 is transferred to the micro-processing unit 13 for being processed after being wave-shaped and buffered by the rotation speed detection circuit 12. The micro-processing unit 13 then outputs a signal to the second control circuit 1122 to conduct the transistor Q1 and to drive the photo coupler PC1, and then to conduct the bi-directional thyristor TR1 so that the corresponding phase control can be achieved. The first control circuit 1121 is also matched to drive the AC motor 111 to have the optimal rotation speed.
Although the costs of an AC motor and a matching control circuit are low, the AC motor used in the fanner control system has the disadvantage of small torque. Moreover, the rotation speed is limited by the frequency and pole number, and is difficult to increase the torque and the rotation speed. Contrarily, a DC motor used in the fanner control system has the advantage of large torque. Moreover, the rotation speed of the DC motor is not limited by the frequency. However, the costs of the DC motor and its matching control circuit are much higher than its AC counterpart.